The dot product is an operation on two vectors. There are two different definitions of dot product. Let #\vec(A)=[A_1,A_2,...,A_n]# be a vector and #\vec(B)=[B_1,B_2,...,B_n]# be another vector, then we have 2 formulas for dot product:

It is simply the product of the modules of the two vectors (with positive or negative sign depending upon the relative orientation of the vectors).
A typical example of this situation is when you evaluate the WORK done by a force #vecF# during a displacement #vecs#.
For example, if you have:
Work done by force #vecF#:#W=|vecF|*|vecs|*cos(theta)#
Where #theta# is the angle between force and displacement; the two vectors being parallel can give: